Combine flour and 1/4 cup milk, stirring with a whisk until smooth; stir in remaining milk.
Add milk mixture to mushroom mixture; bring to a simmer over medium heat. Cook 3 minutes or until mixture thickens, stirring frequently. Stir in sherry, oysters, shrimp, salt, and pepper; bring mixture to a simmer. Cook 2 minutes or until shrimp turn pink.
Cut toasted bread in half diagonally; place 2 toast points on each of 4 individual serving plates. Spoon mixture evenly over toast points.